Medical implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, the replacement of ruptured vessels, and the treatment of disease such as vascular disease by local pharmacotherapy, i.e., delivering therapeutic drug doses to target tissues while minimizing systematic side effects. Such localized delivery of therapeutic agents has been proposed or achieved using medical implants which both support a lumen within a patient's body and place appropriate coatings containing absorbable therapeutic agents at the implant location.
The delivery of expandable stents is a specific example of a medical procedure that may involve the deployment of coated implants. Expandable stents are tube-like medical devices, typically made of stainless steel, Tantalum, Platinum, or Nitinol alloys, designed to be placed within the inner walls of a lumen within the body of a patient. These stents are typically maneuvered to a desired location within a lumen of the patient's body and then expanded to provide internal support for the lumen. The stents may be self-expanding or, alternatively, may require external forces to expand them, such as by inflating a balloon attached to the distal end of the stent delivery catheter.
The process of applying a coating onto a stent or other medical device may be accomplished in a variety of ways, including, for example, spraying the coating substance onto the device using conventional gas-assist or ultrasonic atomization, conventional electrospraying, and electrostatic fluid deposition, i.e., applying an electrical potential difference between a coating material and a target device to be coated, causing the coating material to be discharged from the dispensing point and attracted toward the target by an electric field.
Common to these processes is the need to apply and dry the coating in a manner to insure that an intact, encapsulated and robust coating of the desired thickness is formed on the stent. Equally important is the need to control coating uniformity and quality (on the outside coated surface of a substrate, any radial, side-wall surface of a latticed substrate, and/or inner surfaces of a substrate), coating deposition efficiency, and coating droplet or particle size distribution and concentration.
Conventional gas-assist coating methods, such as coating applications utilizing a gas atomization nozzle, have been used to coat medical devices. However, conventional gas-assist coating methods have shown intrinsic problems in adequately controlling coating uniformity and coating quality through the generated coating droplet size distribution and resulting drying time of the coating film, which can affect the kinetic drug release rate in coatings with embedded drug particles. In addition, conventional gas-assist methods delivered by high-velocity gas streams may have low deposition efficiencies with either partial or incomplete deposition or excessive overspraying. In many systems, generally only about 5% of the coating material or solution that is sprayed from a gas atomization nozzle is deposited on a medical device. The remaining 95% of the coating solution is lost in excessive overspraying and is therefore wasted. Deposition efficiencies have become more important as coating materials have increased in cost, and product processing throughput has become limited by the coating efficiency rate.
Conventional electrospraying and electrostatic methods have also been used to coat medical devices. Conventional electrospraying methods can have relatively high deposition efficiency rates, and can adequately control coating uniformity and droplet sizing for electrically conductive coating materials or solutions. However, controlling droplet sizes and maintaining a stable or robust spray coating process within the well-known “cone-jet” mode can become more difficult with coating solutions having a low electrical conductivity. Conventional electrospraying methods use metal capillary tubes which are electrically conductive and either rely on intrinsic charge carriers or dissociation of ions in a conductive solution to achieve the desirable coating performance. As a result, conventional electrospraying methods require a coating material or solution with adequate electrical conductivity, which can be achieved through mixture design or conductivity additives; conventional electrospraying methods may thus be incompatible with insulative solutions. Conventional methods such as gas-assist and electrospraying may also prevent precise control over the trajectory or specific deposition location of the material to be deposited.